The present invention relates to a tuning circuit suitable for use in, for example, radio communication equipment.
Tuning circuits in radio communication equipment often include a voltage-controlled oscillator (VCO) controlled by a phase-locked loop (PLL). The tuning circuit may also include a filter such as a low-pass filter or bandpass filter. The tuning operations include tuning of the oscillation frequency and quality factor of the VCO, and tuning of the cutoff frequency and quality factor of the filter. Frequency tuning will also be referred to below as f-tuning, and quality-factor turning will be referred to as Q-tuning.
Tuning circuits that can be realized as complementary metal-oxide-semiconductor (CMOS) integrated circuits are desirable for miniaturization and economy. One such circuit is described by Bram Nauta in xe2x80x9cA CMOS Transconductance-C Filter Technique for Very High Frequencies,xe2x80x9d IEEE Journal of Solid-State Circuits, Vol. 27, No. 2, 1992. The VCO and filter in this circuit employ transconductance elements with variable transconductance values. The transconductance values are controlled by means of two power-supply voltages that are supplied to each transconductance element. One power-supply voltage is controlled for f-tuning by a PLL that compares the frequency and phase of the VCO output with the frequency and phase of a reference clock signal; the other power-supply voltage is controlled for Q-tuning by a loop that detects the amplitude of the VCO output and compares the amplitude with a reference voltage. These loops tune the frequency and quality factor of the VCO so that the VCO oscillates with the desired frequency and amplitude. The filter is tuned by copying the power-supply voltages supplied to the VCO.
This tuning circuit has excellent high-frequency characteristics, and also includes a temperature compensation function. However, although the Q-tuning control loop operates much faster than the f-tuning control loop, the two loops are not completely independent, and there remains a risk of unstable operation for certain combinations of the tuning parameters.
An object of the present invention is to provide a tuning circuit, using a transconductance element, that remains stable when tuned by feedback control with respect to both frequency and amplitude simultaneously.
The invented tuning circuit includes an oscillator, a frequency control circuit, and an amplitude control circuit.
The oscillator receives an oscillating input signal and a control signal, and generates an oscillating output signal. The frequency of the oscillating output signal is determined by the transconductance of a transconductance element in the oscillator. This transconductance is controlled by the control signal. The amplitude of the oscillating output signal is responsive to the oscillating input signal.
The frequency control circuit detects the phase difference between the oscillating output signal and a reference signal, and generates the above-mentioned control signal.
The amplitude control circuit detects an amplitude limit of the oscillating output signal, and generates the oscillating input signal according to the detected amplitude limit, by amplifying the oscillating output signal with a gain responsive to the detected amplitude limit, for example.
In the invented tuning circuit, control of the frequency of the oscillating output signal is completely independent of control of the amplitude of the oscillating output signal, so the respective control loops are stable under all combinations of the frequency and amplitude control parameters.